Why do fall colors change?

In the midst of summer’s sizzling heat, bright sunshine and rumbling thunderstorms, the beginning forces of fall foliage are underway. “A lot of the influences on fall color will take place or not take place in the next six weeks,” explained Stan Krugman, a life-long forester who provides the U.S. Forest Service with its annual fall foliage forecast.

Forecasting fall foliage

“I follow the weather around the country. I look at the [national] weather forecasts on television. I try to follow it to get an idea of what’s happening,” he explained.

Then, he applies the season’s weather events to a couple of biological processes that kick in at about the time autumn gets underway.

The science

By the time summer begins, most deciduous trees are covered in green leaves. That green color masks other colors, usually shades of yellow.

“As we get into shorter days and longer nights, a number of hormonal things take place in the leaves,” said Krugman.

A layer forms between the leaf and its stem, choking the movement of nutrients and moisture to the leaves. This makes the leaves’ green color fade, exposing the yellow underneath, explained Krugman.

So, that explains some of the yellows that radiate from tree limbs in the fall, but what about the rest of the gem tones?

The second hormonal change brings out those hues, when sugars used by the trees actually create new colors in the leaves.

“They’re not there in the leaf [when it forms], and they’re usually generated in the fall. They are the reds, blues and the purples. The longer the leaves stay healthy, the longer the [sugar] conversion takes,” said Krugman. And, that translates into a longer, more brilliant season of fall foliage.

Meteorology meets biology

It’s those showers, hot days and dry spells during the summer that dictate how long a leaf will hang on to its green, yellow or gem tones in the fall.

“Some years the color can be absolutely fantastic. If there’s a normal winter with adequate moisture, adequate sugar, no severe drought during the summer and a normal wet fall, you get brilliant color. Part of it is that the leaves stay on the trees longer,” explained Krugman.

On the other hand, “severe drought in the summer — if it’s not alleviated by rain in the early fall, you can expect really quite poor color display,” he warned.

He said that’s because the leaves die before they get a chance to display a great deal of color.